The Bundeswehr has made few sudden large changes in protective headgear since it's inception. It has constantly made improvements as time progresses and feedback is applied to the design process. The categories below are very loose as far as time periods go, but they give you an idea what to look for.
The Bundeswehr was formed in 1955, absorbing elements of the BGS (Bundesgrenzshutzes or Federal German Border Guards) and choosing to copy the US M1 Helmet for their troops.
From 1955 to 1958 the German Army used a direct copy of the M1 helmet with a 2 piece liner called the "zweiteilig" . These are a little tough to get your hands on, but are on par with US helmets as far as wear-ability and protection go.
Next came the M60, put into service in 1958. This pattern took the M1 shell and added a M31 style liner to the helmet itself, eliminating the separate US style liner completely. Here are pictures of 2 M60s, with the first and second pattern liners:
On the outside, they are nearly identical to the US M1. The only way to spot these (and later pattern German Helmets) are the Rivets which attach the liner to the shell.
Once you see the inside, however, these are clearly different form the US M1 and direct M1 copies (such as the Danish and Belgian M1 clones)
In 1961 the Bundeswehr introduced the M1A1 LL , the "Luftlandestruppenstahlhelm". Designed for parachute operations, this helmet has an improved liner and padding inside, as well as a better chinstrap.
You can clearly see the rivets on the shell, and the padding inside. Also the chinstrap which has 2 additional leather straps that attach to the rear of the helmet to better support it on the wearers head.
Next came the M1A1 M62 Helmet. These Helmets came in 3 shell sizes and are similar in shape to the M60, but feature a "Removable" (Replaceable) liner. This helmet was in service from 1962 until the end of Steel Helmets in Bundeswehr Service in the 1990s:
In February 1981 the model M62 helmet had been in service with the Bundeswehr for nearly 20 years, and in line with the modernization of the Bundeswehr around this date, the m62 was given a new interior, an upgraded 3 point chinstrap, and a unique plastic neck protector. This modified helmet was supplied to all branches of the bundeswehr, and is known as the "M1A1 M81 Modifiziert Stahlhelm".
Here you can clearly see the Neck protector at the Rear of the Helmet.
THe Bundeswehr recognized the shortcomings of the Stahlhelm in the mid eighties and began working on a Kevlar helmet which was introduced in 1992 as the "Gefechtshelme M92".
The Helmet was also issued alongside the new flecktarn helmet cover, the first officially in use with the Bundeswehr. Earlier covers were custom made or commercially sourced in Ameoba or Splinter pattern, and there are records of 80's era Bundeswehr troops fitting any foreign covers they could find to their steel helmets (Specifically British DPM and American Mitchell pattern covers).
As for sizes, all steel German helmets have sizes marked in the front of the helmet shell.
Each cover is also sized, and marked to determine whether it fits the steel or kevlar helmet. The helmet covers are reversible, one side Flecktarn, the other "Schneetarn", or snow camo. Helmet covers will be marked on the snow camo side with the size and either an "S" or a "G", for "Stahlhelm" or "Gefechtshelme". Covers marked "S" will not fit the Kevlar helmet, and vise versa!
Note: Newer models of the Kevlar helmet cover will be reversible to "Wustentarn" (desert camo)
This cover is for a steel helmet size 57-61.
The Helmets use European sizing so rough equivalents to sizes would be:
BDU Patrol Cap size US Hat Size European Hat size
X-Small 6 3/4 54
Small 7 56
Medium 7 1/4 58
Large 7 1/2 60
X-Large 7 3/4 62
This is a very rough estimate, best way to make sure your helmet will fit is to measure your head in centimeters. When searching for these helmets, don't forget to try the search term "paratrooper" in addition to your search terms. These regularly show up listed as that for whatever reason.
All photos courtesy of warrelics.eu.